摘要:

The poor rolling/sliding wear performance of transformation‐toughened ceramics such as partially stabilized zirconia (PSZ) is believed to be a result of the phase transformation of precipitates located immediately beneath the contacting surface. These very precipitates, on the other hand, enhance the toughness of the ceramic by preventing the growth of surface cracks. In order to find a compromise solution, an optimization problem is formulated for an idealized model consisting of an edge crack normal to the contacting surface and a periodic distribution of transformable grains in the layer immediately beneath this surface. The objective is to maximize the crack tip shielding by varying the volume fraction and size of the layer without exceeding a prescribed allowable surface uplift. It is shown that maximum volume fraction is attained at a certain depth below the surface but that its magnitude is considerably smaller than that pertaining in peak‐aged

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08353.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The fine particle size fraction from a South China china‐stone material used in porcelains and renowned for its plasticity was characterized to determine the chemical and crystallographic structure of the<2 μm fraction. Chemical analysis suggested the presence of a chloritic material in the fine fraction, a premise which was supported by X‐ray diffraction and thermal analysis. The chinastone has a well‐defined 1.4‐nm basal spacing which did not expand when subjected to ethylene glycol. However, the (001) reflections produced an anomalous broadening and decrease in spacing to 1.28 nm when heated to 700°C. The X‐ray patterns and chemical composition are similar to those at an aluminum‐treated montmorillonite which contains condensed gibbsite interlayered between montmorillonite units. Chemical analysis of individual particles plus X‐ray and thermal analysis leads to the conclusion that the fine fraction of the chinastone is an alumina‐rich chlorite with unusually low iron and

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08354.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Brazing experiments were performed at 750°C for 2 h between Ag–Cu–In–Ti alloy and SiCw/Al2O3. The first clearly nonbraze layer consists of an oxide layer of metallic composition 33Ti–31Al–22Cu–14Si. Areas adjacent to the SiC whiskers were of different composition. A thin, continuous layer on the alumina portion of the composite appears to be γ‐TiO. The SiC whiskers are preferentially consumed and undergo reductions in diameter of approximately 40%. Observed “knobby” whisker morphologies may be related to SiC stacking faults, η‐type phases detected near the Ag–Cu eutectic portion of the joint appear to consist of T

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08355.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

For a wide range of reagent concentrations, particles formed from the hydrolysis of tetraethyl orthotitanate in aqueous ethanol solutions fall into two morphological types. In the first, the particles are composed of uniform spheres, while in the second, the precipitate consists of agglomerates of discrete particles which have been fused together by further reaction products. To explain this observation, we develop a growth mechanism where colloidal stability is imparted to the growing particles through a combination of electrostatic and short‐range repulsions giving rise to a shallow secondary minimum which develops for certain particle sizes and results in the formation of doublets that can be fused together. If the particle surface potential is of sufficient magnitude, the secondary minimum is no longer accessible by Brownian motion and uniform precipitates result. The effects of particle surface potential and reaction medium ionic strength are discusse

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08356.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Aluminum powder entrained by ammonia‐containing nitrogen gas was reacted at various temperatures and time to form aluminum nitride powder. The kinetics of nitride formation were determined by a quantitative X‐ray analysis and compared with those determined by a nitrogen analysis of the product. The conversion to aluminum nitride increased with the reaction time following a sigmoidal rate law. The reaction time for full conversion decreased as the temperature increased in the range 1050°–1300°C. The reaction rate constant at a given temperature was evaluated using the Avrami equation. The activation energy for the reaction was 1054 ± 91 kJ/mol in the temperature range of 1050°–1200°C, and decreased to 322 ± 70 kJ/mol above 1200°C. Comparative analysis of powders formed below and above 1200°C suggested strongly that the rate‐controlling step changed from chemical reaction to mass trans

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08357.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The isothermal oxidation of ZrC single crystals with (100) orientation was carried out at temperatures of 500°, 550°, and 600°C at an oxygen pressure of 2.6 kPa for times up to 240 h. A polished cross section of the oxidized crystal was observed by backscattered electron imaging in a scanning electron microscope. Quantitative chemical analysis for Zr, O, and C and their elemental profiles by the linescan method in the ZrC and oxide scale were performed by wavelength dispersive X‐ray microanalysis. A thin foil of a crystal oxidized at 600°C was examined by transmission electron microscopy. It was found that the oxide scale was divided into two regions, zones 1 and 2, which contained 14 to 23 and 7 to 10 at. % carbon, respectively. Zone 1 exhibited an almost compact, pore‐free matrix ofc‐ZrO2. In zone 2, some growth and aggregation of thec‐ZrO2occurred, producing 5‐ to 20‐nm‐sized particles between which carbon should have been present. The thickness of zone 1 increased parabolically up to 240 h at 500°C and probably in an early period at 550° and 600°C, reaching a constant (about 2 (μm), in contrast to the thickness of zone 2, which increas

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08358.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The effect of spray‐dried granule strength on the micro‐structure of green compacts obtained by isostatic pressing was quantitatively analyzed. The fracture strength of single granules of Si3N4powder made with ultrafine A12O3and Y2O3powders was measured directly by diametral compression. It was found that fracture strength increased notably with the increasing relative density of the granule and the decreasing size of agglomerates in suspension before spray‐drying. Even when green bodies were prepared at an isostatic pressure of 200 MPa, intergranular pores, which negatively affected densification of the sintered bodies, occurred between unfractured granules. The volume and size of these pores in the green compacts increased with the increasing fracture strength of the granules. In the case of closely packed granules, an isostatic pressure of 800 MPa was required to completely collapse the intergranular pores. A simple equation was derived to calculate the isostatic pressure necessary for complete collapse of intergranular pores in the green compacts, and it was determined that granule strength must be kept as low as possible to obtain uniform green com

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08359.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The microstructure of (Sr,Ca)TiO3capacitor‐varistor materials has been investigated by employing electron microscopy techniques (TEM, STEM, HREM, EDX, and EPA). The material is found to contain (Sr,Ca)TiO3grains (∼30 μm) having perovskite crystal structure with domains, a Na+‐diffused layer at the grain boundaries which is dependent on thermal diffusion conditions, and multiple‐grain junctions in which the TinO2n–1Magneli phase coexists with an amorphous intergranular phase. In addition, wider grain boundaries (10–30 nm), thin grain boundaries (∼1 nm), and clean grain boundaries which are free from intergranular phase were observed, and the effects of different grain boundaries on the diffusion of Na

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08360.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

β‐SiC powder containing 6 wt% A12O3and 4 wt% Y2O3as sintering additives was pressureless sintered at 2000°C for 1 h (AYE‐SiC) and 3 h (AYP‐SiC). AYE‐SiC consisted of an equiaxed grain structure and AYP‐SiC exhibited a micro‐structure with platelike grains as a result of grain growth related to β→α phase transformation during sintering,R‐curve behavior and flaw tolerance for these silicon carbides were evaluated by the indentation‐strength technique. For comparison, theR‐curve behavior of conventional sintered, boron‐ and carbon‐doped SiC (SS‐SiC) was evaluated. AYE‐SiC and AYP‐SiC exhibited risingR‐curve behavior with toughening exponents ofm= 0.042 andm= 0.135, respectively. AYP‐SiC exhibited better flaw tolerance and more sharply risingR‐curve behavior than AYE‐SiC. The more sharply risingR‐curve behavior and the better flaw tolerance of AYP‐SiC were attributed mainly to grain bridging of crack faces by platelike grains. Because of the high degree of transgranular fracture, SS‐SiC exhibited a flatR‐curve despit

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08361.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Homogeneous Y‐Si‐O‐N glasses containing 15 or 20 eq% nitrogen (N) were prepared from compositions with Y/Si ratios in the vicinity of that of the lowest eutectic point on the Y2O3–SiO2phase diagram. The liquidus on the phase diagram shifted toward lower temperatures by incorporation of N. The density, the elastic moduli, and the glass transition temperature of the Y‐Si‐O‐N glasses increased with incorporation of N. This is due to the closer packing of atoms in the glasses by the substitution of N, which is in three‐fold coordination with Si, for O which is in two‐fold coordination, and the stronger covalent nature of the Si–N bond compared with the Si–O bond. The coefficient of thermal expansion of the Y‐Si‐O‐N glasses increased with increasing Y content, because the discontinuity of the glass network developed with increasing nonbridging anions by the introduction of Y. In contrast, the glass transition temperature and the elastic moduli increased with Y content due to the high coordination of Y for O, and the relatively high cationic field strength of Y. Furthermore, the effect of cationic field strength on properties of Ln‐Si‐O‐N glasses (Ln

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08362.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY